| Cover | Aug. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 4, 2021 |
| Entity File Number | 001-37852 |
| Entity Registrant Name | PROTAGONIST THERAPEUTICS, INC. |
| Entity Central Index Key | 0001377121 |
| Entity Tax Identification Number | 98-0505495 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | Protagonist Therapeutics, Inc. |
| Entity Address, Address Line Two | 7707 Gateway Blvd. |
| Entity Address, Address Line Three | Suite 140 |
| Entity Address, City or Town | Newark |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94560-1160 |
| City Area Code | 510 |
| Local Phone Number | 474-0170 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.00001 |
| Trading Symbol | PTGX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | true |
